用列表的第N个值统一术语

Question

I'm having trouble implementing a Prolog predicate, vector_reference(L,N,V) , so that when given a list "L" and an index "N" it will unify V with the Nth value of the list.

The example print outs are below:

?- vector_reference([2,3,4], 3, V).
V = 4

?- vector_reference([2, [2,3],’hello’]),2,V).
V = [2,3]

Any help would be GREATLY appreciated.

Answer 1

In the standard library, library(lists) (autoloaded, you don't need to do anything) there are two predicates: nth0/3 and nth1/3 that do exactly what you need. For example:

?- nth1(2, [2,[2,3],hello], V).
V = [2, 3].

Btw, the SWI-Prolog webpage has a very useful search function. Typing nth in there would have given you the answer.